1. Field of the Invention
The present invention is concerned with electronic braking control systems for motor vehicles.
2. Discussion of the Background
Some modern proposed electronic braking control systems have a number of brake pressure control valves distributed around a vehicle, with a single controlling computer responsible for determining the appropriate brake pressure for each wheel or group of wheels on the vehicle.
A brake pressure control valve may consist of a number of solenoids, and possibly brake pressure and other transducers. This entails a large number of connections to the valve. The length and complexity of the wiring to the valve can be reduced by providing a small computer close to, or physically within the valve. This local computer can receive instructions from the master controller via a communications link, and control the operation of the solenoids within the valve to produce the brake pressure changes demanded by the master controller. The local controller can also process signals from the transducers associated with each valve, and pass this data back along the communications link to be master controller.
If these braking control systems are to perform anti-lock braking functions, then wheel speed sensors are necessary at some or all of the vehicle wheels. In some systems, these sensors are connected directly to the master controller. However, where there is a computer in the local brake pressure control valve, the signals from the wheel speed sensors are in some cases taken to this local controller rather than the master. This local computer converts the stream of pulses from the, for example, variable reluctance transducer into a digital signal which is passed to the master controller along the communications link. This reduces the length of the wiring from the sensor, and also the processing load on the master controller.
An example of a known braking system for a four wheel vehicle consists of four brake pressure control valves, one at each wheel, with the wheel speed sensor at each wheel providing a signal to its local brake pressure control valve. Each valve contains a small local computer based controller which converts the pulses from the wheel speed sensors into digital values and passes these along a communications link to a central master controller. This master controller is connected to a transducer at the brake pedal which indicates the driver's brake demand. The master controller uses the driver's brake demand to calculate an appropriate brake pressure for each wheel, and instructs each valve to produce this pressure at its brake. The valve controllers compare the brake pressure demanded by the master controller with that measured by a transducer connected to the local brake, and generate the appropriate solenoid activity to bring the local brake pressure into line with the demand from the master controller.
In the event of a wheel skidding because of excessive brake pressure being applied, anti-lock brake control must come into operation. Anti-lock brake control is somewhat empirical in nature, and so there are many different ways in which good control can be achieved. An example algorithm for an Anti-lock Braking System (ABS) is organized into 3 states (FIG. 5). At any time, each wheel will be designated as being in one of the three states, and the brake pressure at that wheel will be controlled by logic associated with that particular state.
State 0 (box 60) is the skid detection state. When a wheel is not skidding, its measured speed will be close to the speed of the vehicle on the road, and the wheel will be in state 0. While in state 0, the speed and acceleration of the wheel are monitored. When a wheel begins to skid, it will normally decelerate at a rate which is greater than the maximum rate at which the vehicle can decelerate. This deceleration can be detected and taken as an early indication that a skid is imminent. As the skid develops, the speed of the wheel will fall substantially below the speed of the vehicle on the road. If the vehicle speed is known, the difference between vehicle speed and wheel speed (wheel slip) can be used as another indication that a skid is developing. When some predetermined combination of wheel slip and deceleration have been seen, the wheel will move into state 1 (box 62). This transition occurs at point A on FIG. 2.
State 1 is the pressure dump state. When a skid has been detected and the wheel has moved into state 1, the pressure in the brake associated with that wheel will be reduced at a rapid rate. Normally, the braking medium (hydraulic fluid or air) is released through a restriction, so the rate at which the pressure drops will depend on the pressure in the brake. The appropriate duration for the pressure dump will depend on the road surface over which the wheel is running. On a high friction surface, the brake pressure will be high and so the pressure dump rate will be high. Also the wheel will recover from a skid while there is still a substantial pressure in the brake. In this case a very short dump will be appropriate. On low friction surfaces, the dump rate is much slower, and the pressure must be dropped to a very low value before the wheel will recover from the skid. In this case the pressure dump must be much longer. The decision to terminate the pressure dump will therefore depend on a judgement of surface friction. On high friction surfaces, the pressure dump might be terminated while the wheel is still decelerating, but the rate at which it is decelerating is reducing. On lower friction surfaces, the pressure dump might continue until the wheel starts to re-accelerate, or even until the wheel is running once again at vehicle speed. When the conditions for termination of the pressure dump have been met, the wheel will move into a state 2 (box 64). This transition occurs at point B on FIG. 2.
State 2 is the wheel recovery monitoring state. When signs of wheel recovery appropriate to the surface have been seen and the pressure dump has been terminated, the brake pressure will be held at a constant level while the wheel recovery is monitored. On low friction surfaces, the wheel skid will be almost over before the wheel moves into state 2, but on high friction surfaces, the wheel speed might still be reducing. In either case, the brake pressure should have been reduced enough to allow the wheel to recover from the skid and return to running at vehicle speed. The logic associated with state 2 generates expected values for wheel speed and acceleration which will, if followed, brings the wheel back up to vehicle speed in an acceptable time. If the wheel speed and acceleration do not meet these expected values, then the wheel will move back into state 1 and further pressure will be dumped. If the wheel recovery continues satisfactorily, a decision must be taken about when to begin to re-apply the brake pressure. As with the decision to terminate pressure dump, this will depend on surface friction. On high friction surfaces, time spent with brake pressure lower than normal will result in longer stopping distances and so brake pressure should be re-applied as soon as it is clear that the wheel will recover from the skid. On very low friction surfaces, beginning pressure re-apply early can prevent the wheel returning to vehicle speed at all, which will have a detrimental effect on steering control and vehicle stability. In this case brake pressure should not be re-applied until the wheel is already running at vehicle speed. The decision to re-apply brake pressure will be based on a judgement of surface friction and on some combination of wheel speed and wheel acceleration behavior. When this condition is met, the wheel will move back into state 0. This transition occurs at point C on FIG. 2.
When the wheel is back in state 0, its speed and acceleration are monitored to detect skidding as before. The wheel having just recovered from a skid, the state 0 logic also controls the way in which brake pressure is re-applied. Minimum stopping distances are achieved by controlling the pressure in the brake at a high level on average. The brake pressure should therefore be maintained just below the level which will cause the wheel to skid (skid-pressure). Since this pressure can, in practice, change during a stop, the appropriate level of braking cannot be known, and so brake pressure must be re-applied until the wheel is made to skid again. On high friction surfaces, the brake pressure is often dumped to a level far below skid pressure, and so re-apply must be rapid in order to restore a high level of braking. When the pressure in the brake approaches skid-pressure, the re-applying rate should be reduced to delay the next skid, and so maximize the time spent with the brake pressure at a high level. On low friction surfaces, the wheels will skid at very low brake pressures, and so a very slow brake apply should be used from the start. Many electronic braking systems do not have any means of measuring brake pressures, so the decisions about pressure re-applying must be taken on the basis of a judgement of surface friction and a record of past ABS control activity. In systems which do have direct measurement of brake pressures, both the skid-pressure and the brake pressure during re-applying can be measured, and so decisions about pressure re-applying can be simplified.
Many of the decisions described above are taken based on a knowledge of vehicle speed or surface friction. Most electronic braking systems do not measure these variables directly. Vehicle speed is estimated by monitoring and filtering the speed signals from all of the road wheels. When wheels are skidding, this estimate can be improved by using a measurement of vehicle acceleration from a chassis mounted accelerometer if this is fitted as part of the system. Surface friction can be judged from vehicle-wide behavior, such as vehicle deceleration, or from the behavior of an individual wheel, such as re-acceleration rate after a wheel skid or skid-pressure if this can be measured. The information used to judge surface friction may be different for different decisions within the ABS logic.
In order to perform all of the functions described above, an anti-lock braking program is large and complex and requires a powerful computer to execute it.
In some known systems, the control of brake pressure in response to wheel skidding is performed entirely by the master controller. This takes advantage of the fact that the master controller can observe the behavior of the whole vehicle and can generate signals relating to the whole vehicle, such as vehicle speed. A disadvantage with this system is that there is a delay imposed by the need to communicate wheel speed information to the master controller, and later communicate brake pressure demands back to the local valves. This slows the response of the system to sudden skids, which can reduce the level of performance, particularly on low friction surfaces.
In other known systems, the control of brake pressure in response to wheel skidding is performed entirely by the local valve controllers. In such a system, there may even be no master controller at all. This system has the advantage of speed of response, but it requires all of the local valves to have access to vehicle-wide variables, and to contain the full ABS logic which uses this data. This creates a greater load for the communications bus, increases the complexity of the software controlling the communication, and demands a greater processing power from the local controllers, possibly requiring more expensive computers to be used.